1. Field of the Invention
The present invention relates to a hull form of a vessel, and more particularly to a structure of a stern part of a vessel.
2. Description of the Related Art
It is well known that a conventional single screw vessel is equippedwith a propeller shaft located on the vertical central plane of the hull, regardless of being symmetrical about the hull center line or not. Water-flows, generated under the stern part of such a symmetrical vessel, flow into propeller blades, downwards from above and inwards from outside, symmetrically with regard to the propeller shaft of the vessel, to form vertical vortices around longitudinal axes. Water-inflows to the propeller blades, in the case of an asymmetrical type vessel, form asymmetrical flows. Thus, complicated distribution of wakes is generated during sailing.
Large vessels with high block coefficients and wide breadth have been increasing in number in order to improve loading capacity. Because of the high blockage coefficient and the wide breadth, vertical vortices around longitudinal axes generated on a propeller disc plane from the aforementioned wakes, have increased even more. Those vertical vortices are generated in pairs at both vessel sides, causing the wakes to be unbalanced on the propeller disc plane. This results not only in reducing propulsive efficiency but also in increasing hull resistance.
In these circumstances, there have been demanded improvement in loading capacity and, at the same time, in reducing fuel consumption for sailing. To satisfy both demands, improvement in propulsive efficiency has become an indispensable requirement. Occurrence of vertical vortices, which causes reduction of the propulsive efficiency of a vessel, has been unavoidable to a vessel having a large load capacity. To overcome this problem, arrangements of water-inflow directions to propeller blades have been taken, depending on a type of a vessel.
Japanese patent examined publication (KOKOKU) No. 37315/72, for example, descirbes a method wherein:
(1) A propeller shaft of a vessel is positioned on the vertical central plane of the hull, and the stern end edge of the vessel is positioned, by far, eccentrically from said vertical hull center plane;
(2) The distance between the vertical center plane and the line passing through the stern and edge is larger than the radius of the propeller disc plane which the propeller blades form by rotation, and the propeller disc plane is positioned, on one side of the vessel, completely off the vertical hull center plane; and
(3) The generated vortices flow in the reverse direction of the propeller blade rotation.
This method, however, has the following drawbacks:
(a) The distance between the line passing throught the stern end edge and the vertical hull center plane is required to be longer tha the radius of the propeller disc plane, consequently, the distance from the vertical hull center plane to the stern end edge is forced to be large. This results not only in raising ship-building cost but also in affecting unfavorably the steering ability of a vessel.
(b) The shape of one side of the stern part becomes extremely slender in space. Resultantly, installation of engine machinery is tightly restricted, and transportation efficiency is badly affected.